1. Field of the Invention
The present invention relates to a stereoscopic display apparatus and, more specifically, to a stereoscopic display apparatus which allows a viewer to see a stereoscopic image or three-dimensional image without wearing special glasses.
2. Description of the Related Art
Among conventional stereoscopic display apparatuses for displaying a stereoscopic or three-dimensional image are an apparatus which provides pieces of image information having a binocular parallax independently of each other before the right and left eyes, an apparatus which sequentially displays sectional images of an object in synchronism with movement of a mobile screen, and an apparatus using a hologram. However, the apparatus which moves a screen and the apparatus using a hologram have difficulty displaying a moving image because of a large amount of information thereof. For this reason, at present, the development of stereoscopic display apparatuses is mainly directed to the type which displays a stereoscopic image by using binocular parallax images.
Constructed by using a conventional display device such as a cathode-ray tube (CRT) or a liquid crystal display, the stereoscopic display apparatus which displays a stereoscopic image by using binocular parallax images can produce a stereoscopic image relatively easily. In one type of stereoscopic display apparatus utilizing the principle of binocular parallax, a viewer wears special glasses such as polarizing glasses or liquid crystal shutter glasses. In the other type, a viewer sees a stereoscopic image through a special optical element such as a lenticular lens or a parallax barrier serving as a parallax image separating means which element is provided on a display surface.
Of the above two types of apparatuses, the latter type has an advantage, over the former type, of allowing a viewer to see a stereoscopic image without wearing special glasses because parallax images are separated from each other spatially and provided before the respective eyes independently. Therefore, the latter type of stereoscopic display apparatus not requiring the use of glasses will be described below taking, as an example, a type which uses a lenticular lens as the parallax image separating means.
FIG. 1 schematically illustrates the configuration of an example of a conventional lenticular-type stereoscopic display apparatus using a lenticular lens. As shown in FIG. 1, a conventional lenticular-type stereoscopic display apparatus 1 is constituted of a display device 2 and a lenticular lens 3. While the apparatus 1 is of a stereogram type which uses two kinds of parallax images, i.e., a right-eye image R and a left-eye image L, a panoramagram type apparatus which uses three or more parallax images may be constructed in a similar manner. The lenticular lens 3 is composed of a number of long and narrow cylindrical lenses arranged in one direction.
The focal plane of the cylindrical lenses is made a display surface of the display device 2, and a stereoscopic image is displayed on the display surface. A stereoscopic image is formed by dividing a right-eye image R and a left-eye image L each bearing binocular parallax information into vertically long striped images in accordance with the number of cylindrical lenses, extracting those images, and then rearranging those images alternately. In displaying a stereoscopic image, the positions of the display device 2 and the lenticular lens 3 are adjusted so that divided right-eye images R and left-eye images L are displayed at locations that are determined by the positions of the cylindrical lenses and a right eye 4 and a left eye 5 of a viewer.
When the viewer watches this lenticular-type stereoscopic display apparatus 1, the right eye 4 sees only the right-eye images R and the left eye 5 sees only the left-eye images L. The viewer can recognize the displayed image as being stereoscopic by fusing the images R and L within his brains.
FIG. 2 schematically illustrates viewing regions of the conventional lenticular-type stereoscopic display apparatus 1. In the lenticular-type stereoscopic display apparatus 1 constituted of the display device 2 and the lenticular lens 3, right-eye images R and left-eye images L that are displayed on the display device 2 are converged at positions that are distant from the stereoscopic display apparatus 1, to form right-eye image regions VR.sub.R and left-eye image regions VR.sub.L. That is, if the stereoscopic display apparatus 1 is watched by one eye from one of the right-eye image regions VR.sub.R, only the right-eye images R are seen over the entire screen. Similarly, if the stereoscopic display apparatus 1 is watched by one eye from one of the left-eye image regions VR.sub.L, only the left-eye images L are seen over the entire screen. Therefore, if the right eye 4 and the left eye 5 of the viewer 6 are located in a right-eye image area VR.sub.R and a left-eye image area VR.sub.L, respectively, the viewer 6 can recognize a stereoscopic image by fusing those images within his brains.
The head of the viewer 6 is positioned such that first entirely different, i.e., distinguishable, right-eye and left-eye images R and L are provided. However, the width of the right-eye image regions VR.sub.R and the left-eye image regions VR.sub.L is very small, i.e., smaller than the eye span of the viewer 6 which is 60-65 mm. Therefore, the viewer 6 is obliged to watch the apparatus 1 with his head fixed in a state that the right eye 4 and the left eye 5 are located in a right-eye image region VR.sub.R and a left-eye image region VR.sub.L, respectively.
If the right eye 4 and the left eye 5 of the viewer 6 are located in a left-eye image region VR.sub.L and a right-eye image region VR.sub.R, respectively, not only is stereoscopic vision disabled but also there occurs inconsistency between stereoscopic information obtained from binocular parallax and that obtained from planar images that are seen by the respective eyes (see "The Journal of Three Dimensional Images", Vol. 7, No. 2, March 1993). This type of inverse stereoscopic vision (pseudoscopic image) makes the viewer 6 feel uncomfortable and resultingly feel fatigued. In general, right-eye and left-eye images are different from each other only slightly (i.e., a parallax is very small), they are difficult to distinguish. Therefore, there may occur a case that the viewer 6 watches the apparatus 1 for a long time in the state of inverse stereoscopic vision and is fatigued cumulatively.
In summary, in the conventional lenticular-type stereoscopic display apparatus, right-eye image regions VR.sub.R and left-eye image regions VR.sub.L are arranged periodically at the viewing position of the viewer 6 and their width is very small. Therefore, if the viewer 6 moves his head only slightly, the state of inverse stereoscopic vision is established to make him feel fatigued. Further, once the head position is deviated, the viewer 6 cannot know the correct position any more, so that he may watch the apparatus 1 for a long time in the state of inverse stereoscopic vision.
The above problems are common to stereoscopic display apparatuses which allow a viewer to see a stereoscopic image or three-dimensional image without using special glasses by spatially separating parallax images and providing separated images independently before the respective eyes, such as an apparatus using a projection-type display device and a lenticular lens, an apparatus using a plurality of projection-type display devices and two lenticular lenses, and an apparatus using a parallax barrier as a parallax image separating means in addition to the above-described stereoscopic display apparatus which uses a direct-view display device and a lenticular lens.